Monday, November 22, 2010

Updated Windsurf Calculator Online

When I realized I could embed an Excel file on my blog I decided to do it with the latest version of my Automatic Windsurfing Equipment Calculator. Heh heh heh. Some changes from the old version of the calculator are: 1) Fixed the lbs / kgs typo, 2) Added a decimal place to the wind strengths table, 3) Simplified the fin size calculator, 4) Added several more categories of board to the board size calculator. Let me know what you think.

That's actually a good question. A shortboard is any board that lacks a daggerboard and is designed primarily for planing. Based on those criteria your Carve 145 is definitely a shortboard. Even a big formula board is considered a shortboard.

For better or worse, it's all about weight. Girls usually don't need as big sails as boys, but that's only because girls are usually lighter. If a girl and a guy weighed the same, they would need the same size sails to plane.

The calculator shows that pretty well. A 6.5 to a 132 pound person has the same power as a 9.8 to a 200 pound person- both would get their riders going in about 12 knots.

If the Fanatic works well with your biggest sails then there's probably no need to hang on to the Hifly. You'll have a blast on the Rocket 105 when it's windy, and the sailing style for a Rocket is relatively similar to the freeride boards you're used to so it won't be a huge transition. At some point, though you might want to think about what style of highwind windsurfing you want to do. If you want to blast around and occasionally catch air, then the Rocket is good. If you want to play around with carving and jumping in the waves and chop then a freestyle wave board would be better.

Hi ElTurco- Thanks! The formulas aren't based on rigorous physics. They're just equations to fit a trend curve to the empirical data. I got the original sail size formula off the Starboard Windsurfing forums, and adapted it from there. The fin size and board size equations I came up with myself.

I wonder if this needs an update for the new WindSUP Boards. I am getting a Exocet WindSUP 10' tomorrow and I am hoping to plane with my 7.5 sail in 12mph average wind. I am 155lbs.

I am pretty close to planing with my BIC WindSUP 10'6" but the BIC is not known as a good planing board so I am hoping the foot straps and the step tail and the shape give me that extra speed.

Also with the WindSUP boards it is not easy to tell when you are planing or fast gliding in my opinion. With my shorter board I am either in the foot straps and going fast on a plane or I am slogging and not able to step into the foot straps. The WindSUP has a pleasant fast glide which is kind of like planing, but not as fast I suppose.

I'm not sure if maybe my wind meter is out, but I find I am getting more realistic answers when I input my weight as 70 Kg, whereas I am actually 92 Kg! I do have (for the bigger sail sizes, which is where my interest is concentrating) a 2014 Fanatic Gecko 120 (77 wide) which planes unbelievably early, is that perhaps skewing things?

Hi Chris- An efficient board and good skills will definitely help. You probably have both of those things. Also, keep in mind that the calculator isn't designed to tell you the bare minimum necessary to plane. The number the calculator spits out is what's *ideal*; what will get you comfortably planing with some power to spare. So your absolute limit planing threshold might be around 2 knots lower than what the calculator spits out. Also, the wind is often less on the shore where you use your wind meter than out in the open water where you're sailing.

Why would fin size vary with sail size? Here's my thinking: the fin exists to offset the sideways thrust of the sail. The sail's overall thrust depends on size and windspeed. I chose sails based on windspeed in a way that approximately cancels the effect of windspeed. The fin's offsetting force is roughly determined by board speed. But, my sail selection also has the effect that my typical board speed is largely independent of wind speed. Therefore, I generally use the biggest fin that I can handle given the width of my board (and my typical top speed). Am I missing something?

My recommended long board fin was 40cm. I ended up putting a 50cm fin on the board to get more low end power in light winds and more top speed. Everyone recommended around 44cm, but Maui Fin suggested 50cm based on what I wanted to do with light wind planing, and I am very happy with this size fin and run with it all the time, even in high winds.

Hi Eric- I understand what you're saying. To make sense of it you have to think of component vectors. The force on the sail has both a forward thrust component (think of an arrow pointing towards the nose of the board) and a lateral pressure component (think of an arrow pointing downwind). The fin forces include drag force (think of an arrow pointing towards the rear of the board) and lateral resistance (think of an arrow pointing upwind). A small sail in high winds and a large sail in light winds provide the same amount of forward thrust... but different amounts of lateral pressure. The large sail in light winds exerts more lateral pressure relative to the forward thrust it provides, which is why you need to pair it with a larger fin to cancel out that lateral pressure vector arrow. Conversely, using a large fin with a small sail in high winds is overkill- you have more lateral resistance that you need, and you also have unnecessary drag.

Sail thrust is more or less perpendicular to the apparent wind (if you ignore the sail drag and assume optimum angle of attack). As you note, we usually project that onto the direction of forward motion (your forward thrust component) and onto the lateral (actually downwind if you don't want a negative number). Those amplitudes relate to the overall thrust vector via cos and sin of the angle of the sail vs the centerline. Essentially, those forward thrust and lateral thrust components are controlled by the position of the boom. (We've simplified this by imagining it's all in the plane of the board.) Really, we model this as a vector at the sail's center of effort. I think we as riders intuitively position that center of effort right over the board's point of rotation. In other words, even tho the center of effort generally moves up and back for a larger sail, I think we intuitively adjust the larger sail position the center of effort the same (except for height, which I think we compensate for by leaning the sail top windward) and angle the thrust vector the same (otherwise, we'd be rig steering). This model, if correct, just leaves the fin a tradeoff between drag and up-wind ability.

What weight should you input for the calculator? "Natural" or Equipped? The weight difference between a person in the nude and a person completely dressed in their windsurfing gear (Wetsuit, harness, life jacket/impact vest, booties, gloves) can be quite a bit, especially if the gear is also wet! This difference could significantly affect the results of the calculator. I'm legitimately curious because I sail in an area where this gear is required most of the year.

Ann, in my experience, I adjust the weight in the table until the readings appear closer to my actual experiences out on the water. Having done that, I would say that you will get more accurate results using your "ready for battle" all-up weight. I do find however that people's experiences of wind speed measurements do vary, depending on where you measure, the calibration of the instrument, etc. etc.